1. Field of the Invention
The present invention relates to a planar antenna for use in frequency bands such as millimeter wave and microwave bands, and more particularly to a planar antenna which has a slot line and is capable of controlling an electromagnetic wave field to change an antenna frequency and a plane of polarization and which can be easily designed.
2. Description of the Related Art
Generally, planar antennas are widely used in radio communications and reception of satellite broadcasts as they can easily be processed and small in size and light in weight. The inventors of the present invention have proposed a planar antenna having an array of slot-line antenna elements disposed on a substrate, as disclosed in Japanese laid-open patent publication No. 2004-7034 (JP, P2004-7034A), and have also proposed a microstrip-line planar antenna for controlling an electromagnetic wave field to change an antenna frequency and a plane of polarization, as disclosed in Japanese laid-open patent publication No. 2003-110322 (JP, P2003-110322A).
FIGS. 1A and 1B show a conventional frequency-variable microstrip-line planar antenna. The illustrated microstrip-line planar antenna basically comprises a microstrip-line resonator. The antenna has substrate 1 made of a dielectric material which supports, on one principal surface thereof, resonant conductor 3 and feeding line 2 extending from resonant conductor 3 to an end of substrate 1. Substrate 1 also supports ground conductor 4 disposed on and extending fully over the other principal surface of substrate 1. Each of feeding line 2 and resonant conductor 3 has a microstrip-line structure.
Resonant conductor 3 has an opening 5, which is of a rectangular shape, for example, defined substantially centrally therein, exposing the one principal surface of substrate 1 therethrough. Electronic device 6 is disposed across opening 5 to interconnect opposite sides of resonant conductor 3 which are positioned across opening 5. Electronic device 6 comprises variable-reactance device 6A which may be, for example, a voltage-variable capacitance device whose capacitance is variable depending on a voltage applied thereto. In the illustrated microstrip-line planar antenna, the voltage-variable capacitance device comprises a pair of varactor diodes connected in series to each other with their respective cathodes connected to each other. The anodes of the varactor diodes are connected respectively to the opposite sides of resonant conductor 3 which are positioned across opening 5. Conductive line 7 is connected to the common junction between the cathodes of the varactor diodes. Reverse-biasing control voltage V1 is applied between conductive line 7 and resonant conductor 3.
In this arrangement, when control voltage V1 is changed, the capacitances of the varactor diodes are changed, changing boundary conditions for developing an electromagnetic wave field on resonant conductor 3. In this manner, the resonant frequency (i.e., antenna frequency) of the microstrip-line planar antenna is changed. Stated otherwise, the resonant frequency, i.e., the antenna frequency, can be controlled by changing control voltage V1 applied to the varactor diodes of variable-reactance device 6A.
The same principles are also applicable to a variable-polarization-plane microstrip-line planar antenna shown in FIG. 2. As shown in FIG. 2, the variable-polarization-plane microstrip-line planar antenna includes circular resonant conductor 3 having circular opening 5 defined concentrically therein and switching device 6B disposed across circular opening 5. Switching device 6B corresponds to electronic device 6 of the planar antenna shown in FIGS. 1A and 1B, and comprises four PIN diodes, for example. The four PIN diodes are in a star-connected configuration wherein the diodes in each diametrically opposite pair are connected in reverse polarity. Specifically, the four diodes are connected to common junction O, with the first and third diodes having respective anodes connected to common junction O and the second and fourth diodes having respective cathodes connected to common junction O. Circular resonant conductor 3 is divided into four sectors to define four quadrant points, i.e., left, lower, right, and upper quadrant points, around circular opening 5. The first diode has a cathode connected to the left quadrant point, the second diode has an anode connected to the lower quadrant point, the third diode has a cathode connected to the right quadrant point, and the fourth diode has an anode connected to the upper quadrant point. Feeder 2 extends from an upper right corner as shown of the substrate obliquely downwardly toward the center of resonant conductor 3, and is connected to an outer edge of resonant conductor 3. Conductive line 7 for applying switching control voltage V2 is connected to common junction O.
Resonant conductor 3 shown in FIG. 2 has resonant modes of TM11 which are degenerated in both vertical and horizontal directions. These two resonant modes have the same resonant frequency. When negative control voltage V2 is applied to render the second and fourth diodes in the vertical pair conductive, the vertically resonant mode of the degenerated resonant modes is not excited. When positive control voltage V2 is applied to render the first and third diodes in the horizontal pair conductive, the horizontally resonant mode of the degenerated resonant modes is not excited. Therefore, resonant conductor 3 is resonated in either one of the degenerated resonant modes by selectively turning on the vertical and horizontal pairs of diodes of switching device 6B. In this manner, the planar antenna shown in FIG. 2 is capable of switching between planes of polarization for transmitted and received electromagnetic waves.
With the conventional microstrip-line planar antennas described above, the microstrip-line resonator, i.e., the resonant conductor, has the opening for placing the electronic device for controlling frequencies and planes of polarization. The basic design of the microstrip-line resonator itself is complex because electric characteristics, e.g., the resonant frequency, of the microstrip-line resonator are subject to change depending on the shape and size of the opening. In addition, inasmuch as control voltage V1, V2 is applied from a control circuit (not shown) to the electronic device disposed across the opening, a component such as a choke coil is required to isolate the resonant conductor and the control circuit from each other at high frequencies. Consequently, the conventional microstrip-line planar antennas are made up of a large number of parts, and their control circuits are complex in structure.
Generally, microstrip-line planar antennas have a narrow frequency range, a low antenna gain, and a high radiation level of the cross polarization component from the antenna element. The cross polarization component refers to a polarization component which is perpendicular to the polarization component that is originally intended for transmitting and receiving electromagnetic waves. Another problem is that the feeding line connected to the resonant conductor tends to affect the boundary conditions of the microstrip-line resonator in the vicinity of the junction between the feeding line and the resonant conductor.